A typical motor vehicle is generally characterized as comprising an unsprung mass and a sprung mass. The unsprung mass generally consists of all of the parts of the vehicle not supported by the vehicle suspension system such as the tire/wheel assembly, steering knuckles, brakes and axles. The sprung mass, conversely is all of the parts of the vehicle supported by the vehicle suspension system. The unsprung mass can be susceptible to disturbances and vibration from a variety of sources such as worn joints, misalignment of the wheel, brake drag, irregular tire wear, etc. Because vehicular tires support the sprung mass of a vehicle on a road surface and such tires are resilient, any irregularities in the uniformity or dimensions of the tire, any dimensional irregularities in the wheel rim, and/or any dynamic imbalance or misalignment of the tire/wheel assembly will cause disturbances and vibrations to be transmitted to the sprung mass of the vehicle thereby producing an undesirable vehicle ride, as well as reducing handling and stability characteristics. Severe vibration can result in dangerous conditions such as wheel tramp or hop and wheel shimmy (shaking side-to-side).
It is now standard practice to reduce some of these adverse vibrational effects by balancing the wheel rim and tire assembly by using a balance machine and clip-on lead weights. The lead balance weights are placed on the rim flange of the wheel and clamped in place in a proper position as directed by the balancing machine. The balancing procedure can reduce imbalance in the tire/wheel assembly, however, perfect balance is rarely achieved. Balancing is not an exact art and the results are dependent upon the specific set up of a tire/wheel assembly on a specific balancer at that moment in time. Balancing is an improvement and will reduce the vibration of the tire/wheel assembly in comparison to an unbalanced tire/wheel assembly. However, even perfect balancing of the tire/wheel assembly does not necessarily mean that the tire will roll smoothly. The balancing of the tire/wheel assembly must necessarily be done in an unloaded condition. When the balanced tire is placed on the vehicle, the weight of the vehicle acts on the tire through the interface or contact area of the tire and the road surface which is commonly known as the tire footprint. Irregularities in the tire are common such that even a perfectly balanced tire can have severe vibrations due to non-uniformities in the tire which result in unequal forces within the tire footprint.
A level of non-uniformity is inherent in all tires. In the art of manufacturing pneumatic tires, rubber flow in the mold or minor differences in the dimensions of the belts, beads, liners, treads, plies of rubberized cords or the like, sometimes cause non-uniformities in the final tire. When non-uniformities are of sufficient magnitude, they will cause force variations on a surface, such as a road, against which the tires roll and thereby produce vibrational and acoustical disturbances in the vehicle upon which the tires are mounted. Regardless of the cause of the force variations, when such variations exceed the acceptable minimum level, the ride of a vehicle utilizing such tires will be adversely affected.
Non-uniformity is generally characterized as 1) radial runout or out of roundness, 2) radial force variations, and 3) lateral force variations or conicity. Radial runout is the deviation from perfect roundness of the outer circumference of the tire. For example, the beads of the tire may be not exactly concentric relative to the axis of rotation of the tire or the tread may not be concentric with the beads. Radial force variation is the deviation from spindle load transmitted by a perfect tire during rotation. For example, radial force anomalies in a tire may result from “hard” and/or “soft” spots in the tire due to structural non-uniformities such as inconsistent wall thickness, ply turn-up variations, bead set, ply arrangement and other deviations. Lateral force variation is the deviation from straight tracking during rotation of the tire. For example, lateral force variations can result if the belt package of the tire is axially displaced or conically shaped. While lateral force variations will tend to pull the vehicle to a side of the road, it is primarily the radial force variations, including radial run-out, resulting in the vibration and acoustical effects which degrade the ride of the vehicle.
In a non-uniform tire, the radial run-out, the radial forces, and the lateral forces exerted by the tire will vary or change during its rotation. In other words, the magnitude and/or direction of the radial run-out, and the radial and lateral forces exerted by the tire will depend on which increment of its tread is contacting the surface.
Accordingly, methods have been developed to correct for excessive force variations by removing rubber from the shoulders and/or the central region of the tire tread by means such as grinding. These methods are commonly performed with a force variation or uniformity machine which includes an assembly for rotating a test tire against the surface of a freely rotating loading drum. This arrangement results in the loading drum being moved in a manner dependent on the forces exerted by the rotating tire whereby forces may be measured by appropriately placed measuring devices. A computer interprets the force measurements and grinders controlled by the computer remove rubber from the tire tread. However, grinding of the tire has certain disadvantages. For example, grinding can reduce the useful tread life of the tire, it may render the tire visually unappealing or it can lead to the development of irregular wear when the tire is in service on a vehicle.
While uniformity machines have been relatively successful in reducing the undue vibrations transmitted to the sprung mass of the vehicle by the tires, their complexity, manufacturing cost, and the requirement of trained operating personnel has limited the use of these devices primarily to the manufacturing facilities of the vehicle tire manufacturing companies. This has resulted in improved ride characteristics with respect to the original equipment tires on the vehicle but has done little to maintain the original improved ride characteristics when these original equipment tires are worn or replaced with after market replacement tires. Further, the methods used in uniformity testing usually mount the tire on an axle or arbor for testing rather than on the vehicular wheel rim. Because the wheel rim itself can have dimensional inaccuracies which affect uniformity and the remainder of the unsprung mass of the vehicle can also adversely affect uniformity characteristics, correcting the tires with force variation tire grinding without the tire being mounted on the wheel rim and vehicle on which it is to be used will fail to compensate for the total irregularities of the tire/wheel assembly. Furthermore, these characteristics can change as the tire is worn due to uneven or irregular wear and also normal wear progression.
Balancing of the tires has also been accomplished by using methods other than balance machines and lead weights. For example, Fogal in U.S. Pat. No. 5,073,217 disclosed a method of balancing a vehicle tire/wheel assembly by introducing a pulverulent synthetic plastic material into the interior chamber of the tire wheel assembly. The pulverulent synthetic plastic material has the added effect of compensating for the radial and lateral force variations generated at the tire road interface. The movement of the pulverulent synthetic plastic material within the tire is proportional to the downward force of the vehicle weight and the centrifugal force due to the tire rotation. While the invention disclosed in U.S. Pat. No. 5,073,217 worked effectively on truck tires having a large gross vehicle weight (GVW), the 20–40 mesh size pulverulent synthetic plastic material was found to not work as effectively for passenger type vehicles. The reason for the different performance is that the passenger vehicles have a significantly lower GVW. The movement of the inserted particles is directly related to the downward force on the tire. The weight of a typical passenger vehicle is not sufficient to move the 20–40 mesh size pulverulent synthetic plastic material properly within the passenger tire and was thus unable to effectively equalize the radial and lateral forces.
Therefore, there remains a need in the art for an improvement in reducing radial and lateral force variations at the tire footprint due not only to tire/wheel assembly imbalance, but reducing these force variations beyond the improvement levels available by balancing with only conventional balancing methods, in a manner reducing force variations from other causes as well.